From Einav and Finkelstein (2023):

…while 12.5% ​​of individuals under 65 are uninsured at a point in time, twice as many—one in four—are uninsured at some point over a 2-y period. Moreover, the risk of losing insurance remained virtually unchanged with the introduction of the landmark ACA. Risk of insurance loss is particularly high for those with health insurance through Medicaid or private exchanges; they have a 20% chance of losing coverage at some point over a 2-y period, compared to 8.5% for those with employer-provided coverage. Those who lose insurance can experience prolonged periods without coverage; about half are still uninsured 6 months later, and almost one-quarter are uninsured for the subsequent 2 years.

Despite the passage of ACA, reliable health insurance in the US is really only available to the elderly, veterans and those with stable jobs.